1. Field of the Invention.
The present invention relates to an electronic vibrato system and, more particularly, to a method and means for modulating the frequency of an analog signal with no frequency discrimination and with no corresponding modulation of the amplitude thereof.
2. Description of the Prior Art.
In music, the vibrato effect is caused by modulating either the frequency or phase of an audio frequency signal. The vibrato effect is sensed as a ringing of the audio signal and is quite pleasing to the ear. Thus, the vibrato effect is widely used.
What is generally considered to be the first vibrato system was invented by Donald Leslie, The Leslie system being based on the theory of a rotating speaker. That is, if an audio signal is applied to a speaker which is rotated about an axis perpendicular to its output direction, the speaker physically moves toward and away from a listener and a vibrato effect is achieved. However, since it is not practical to rotate the speaker itself, the same effect is achieved either by directing the output of a speaker through a horn and rotating the horn or by mounting a speaker within a drum having a series of openings therein and rotating the drum. In either event, the vibrato effect is achieved mechanically by rotating a speaker or its equivalent.
While the Leslie speaker system has gained a wide degree of acceptance, numerous problems exist therewith. Because of the necessity of mechanically rotating a speaker or its equivalent, the speaker cabinet must contain a drive motor and belts for interconnecting the motor to the speaker. The motor must be oiled and the belts periodically tightened to prevent slippage thereof. In addition, the cable connecting the speaker system to the audio source is large and cumbersome because of the necessity of conducting power and other control signals to the drive motor. Furthermore, there is always a certain amount of noise associated with the mechanically moving parts and it is difficult to control the speed of the motor accurately.
It is conventional, in Leslie speaker systems, to drive the speakers at either approximately 1 Hz or approximately 6 Hz to achieve either a slow or a fast vibrato effect. However, since the speakers are rotating mechanically, it is impossible to rapidly speed up or slow down the motor when changing the mode of operation. While many songs have been written to take advantage of this time delay in changing vibrato speed, it is generally considered to be undesirable.
Numerous attempts have been made to overcome the problems associated with the Leslie speaker system. Typically, light dependent resistors (LDR's) or thermistors have been used in phase shift circuits which receive the analog signal. By modulating the resistance valves of such LDR's or thermistors, the phase shift of the circuits may be varied to give the desired vibrato effect. However, numerous problems exist with such phase shift circuits too. Such circuits are subject to excessive noise and often produce some degree of undesirable amplitude modulation in addition to the phase modulation. Such circuits are often highly frequency selective so that different frequencies are shifted by different amounts and this is also highly undesirable. The waveshape of the ultimate signal is often severely distorted and such circuits are capable of providing limited frequency shifts in time.
Other solutions to the above problems have been very costly. For example, in order to avoid the frequency selectivity of phase shift circuits, it has been proposed to divide the audio band into many sections and to use separate phase shifters for each section. Or, it has been proposed to cascade many phase shifting networks. However, in these as well as in other approaches, it simply has not been possible to provide a variable or fixed change in frequency, phase, or time of an entire band of audio frequencies without discrimination, without excessive noise, without wave-shape distortion, without amplitude modulation, and at a reasonable cost.